n.a humorous term for the brain, originating around the beginning of the 1900s. A tank is a large container or receptacle for holding a liquid, gas, etc., and as used here it refers to the head. The term think tank, came to be used in the 1950s to refer to a group or organizationinvolved in researching and solving complex political, military or social problems and in predicting and planningfuture developments in such areas.(slang) the brain.